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Every technical discipline has its own
standardized way(s) of making descriptive
diagrams, and instrumentation is no exception.
This chapter will discuss three different types of
instrumentation diagrams:
• Process Flow Diagrams (PFDs)
• Process and Instrument diagrams (P&IDs)
• Loop diagrams (“loop sheets”)
• SAMA diagrams

6.1 Process Flow Diagrams
At the highest level, the instrument technician
is interested in the interconnections of
process vessels, pipes, and flow paths of
process fluids. The proper form of diagram to
represent the “big picture” of a process is
called a process flow diagram. Individual
instruments are sparsely represented in a PFD,
because the focus of the diagram is the
process itself.

One might guess the instrument interconnections
based on the instruments’ labels.
For instance, a good guess would be that the level
transmitter (LT) on the bottom of the knockout
drum might send the signal that eventually
controls the level valve (LV) on the bottom of that
same vessel.
One might also guess that the temperature
transmitter (TT) on the top of the evaporator
might be part of the temperature control system
that lets steam into the heating jacket of that
vessel.

Based on this diagram alone, one would be
hard-pressed to determine what control
system, if any, controls the compressor itself.
All the PFD shows relating directly to the
compressor is a flow transmitter (FT) on the
suction line.
This level of uncertainty is perfectly acceptable
for a PFD, because its purpose is merely to
show the general flow of the process itself,
and only a bare minimum of control
instrumentation.

6.2 Process and Instrument Diagrams
Process and instrument diagrams (P&IDs) lie
somewhere in the middle between process flow
diagrams and loop diagrams. A P&ID shows the
layout of all relevant process vessels, pipes, and
machinery, but with instruments superimposed
on the diagram showing what gets measured and
what gets controlled.
Here, one can view the flow of the process as well
as the “flow” of information between
instruments measuring and controlling the
process.

Now we see there is more instrumentation
associated with the compressor than just
a flow transmitter.
There is also a differential pressure
transmitter (PDT), a flow indicating
controller (FIC), and a “recycle” control
valve that allows some of the vapor
coming out of the compressor’s
discharge line to go back around into the
compressor’s suction line.

Additionally, we have a pair of
temperature transmitters that report
suction and discharge line
temperatures to an indicating
recorder.
Some other noteworthy details
emerge in the P&ID as well. We see
that the flow transmitter, flow

Please note the differences in the instrument “bubbles” as
shown on this P&ID. Some of the
bubbles are just open circles, where others have lines going
through the middle. Each of these
symbols has meaning according to the ISA (Instrumentation,
Systems, and Automation society)
standard:

6.3 Loop diagrams
At the lowest level, the instrument technician is
interested in the interconnections of individual
instruments, including all the wire numbers,
terminal numbers, cable types, instrument
calibration ranges, etc. The proper form of
diagram for this level of fine detail is called a loop
diagram.
Here, the process vessels and piping are sparsely
represented, because the focus of the diagram is
the instruments themselves.

The only type of diagram at a lower level of
abstraction than a loop diagram would be
an electronic schematic diagram for an
individual instrument, which of course
would only show details pertaining to that
one instrument.
Thus, the loop diagram is the most detailed
form of diagram for a control system as a
whole, and thus it must contain all details
omitted by PFDs and P&IDs alike.

The more detail you put into a loop
diagram, the easier it makes the
inevitable job of maintaining that
system at some later date.
An interesting detail seen on this loop
diagram is an entry specifying “input
calibration” and “output calibration”
for each and every instrument in the
system.

Another interesting detail seen on this loop diagram is the action
of each instrument. You will notice a box and arrow (pointing
either up or down) next to each instrument bubble.
An “up” arrow (↑) represents a direct-acting instrument: one
whose output signal increases as the input stimulus increases.
A “down” arrow (↓) represents a reverse-acting instrument: one
whose output signal decreases as the input stimulus increases.
All the instruments in this loop are direct-acting with the
exception of the pressure differential transmitter PDT-42:

6.4 SAMA diagrams
SAMA is an acronym standing for Scientific Apparatus Makers
Association, referring to a unique form of diagram used primary in
the power generation industry to document control strategies.
These diagrams focus on the flow of information within a control
system rather than on the process piping or instrument
interconnections (wires, tubes, etc.).
The general flow of a SAMA diagram is top-to-bottom, with the
process sensing instrument (transmitter) located at the top and the
final control element (valve or variable-speed motor) located at the
bottom.
No attempt is made to arrange symbols in a SAMA diagram to
correlate with actual equipment layout: these diagrams are all
about the algorithms used to make control decisions, and nothing
more.

A sample SAMA diagram
appears here, showing a
flow transmitter (FT)
sending a process
variable signal to a PID
controller, which then
sends a manipulated
variable signal to a flow
control
valve (FCV):

A cascaded control system, where the output of one controller
acts as the set point for another controller to follow, appears in
SAMA diagram form like this:

SAMA diagrams may show varying degrees of detail about the control
strategies they document.
For example, you may see the auto/manual controls represented as separate
entities in a SAMA
diagram, apart from the basic PID controller function.

Showing even more detail, the following SAMA diagram indicates the
presence of set point tracking in the controller algorithm, a feature
that forces the set point value to equal the process variable value any
time the controller is in manual mode:

Types of line in a SAMA Diagram
• Solid lines
represent analog
(continuously
variable) signals
such as
processvariable,
setpoint, and
manipulated
variable.
• Dashed lines represent
discrete (on/off) signal
paths,in this case the
auto/manual state of
the controller
commanding the PID
algorithm to get its
setpoint either from
the operator’s input
(A) or from the process
variable input (the
flow transmitter: FT).

6.5 Instrument and
process equipment
symbols

This section shows
some of the many
instrument symbols
included in the ISA 5.1
standard.
These symbols find
application in Process
Flow Diagrams (PFDs),
Process and Instrument
Diagrams (P&IDs), and
loop diagrams alike.

6.5.2 Process/Instrument line connections

6.5.7 Flow measurement devices
(flowing left-to-right)

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